This question is very similar to
Unlike transplants between identical twins, whose genetic endowment is the same, all patients receiving hearts or other organs must take anti-rejection drugs for the rest of their lives.
(A) Unlike transplants between identical twins, whose genetic endowment is the same
(B) Besides transplants involving identical twins with the same genetic endowment
(C) Unless the transplant involves identical twins who have the same genetic endowment
(D) Aside from a transplant between identical twins with the same genetic endowment
(E) Other than transplants between identical twins, whose genetic endowment is the same
Notice the pattern in these - the questions want you to think you're deciding between "unlike", "besides", "not like", etc. But the key to C in each of these choices is that the other answer choices are all modifiers:
-Unlike water, all passengers
-Aside from a transplant, all patients
-Other than the use of headset telephones, all drivers
And none of those modifiers is logical. The subject of each sentence is a type of person (drivers, passengers, patients) but the modifier describes objects.
In each of these, choice C changes the game - it's not a modifier. Each answer choice C provides an "unless X is true, Y is true" setup in which the underlined portion contains its own subject and verb. So "unless headset telephones ARE USED, all drivers are prohibited..." isn't a modifier-subject-verb, it's two related clauses. That's the difference in these.
Note two other things:
1) Each one tries to get you to decide between synonyms (unlike, besides, not like) - which should indicate to you that you have to find a better decision point. We simply won't become experts on the nuances (if, indeed, any exist) between those extremely-similar terms.
2) Each also involves a modifier-within-a-modifier (the interior modifier starting with "which"). If you note that "which" in each case does properly modify the word before it, you can read as though that modifier isn't even there. Modifiers on the GMAT have two roles - they're either decision points based on which you can eliminate the whole answer (because it's an illogical modifier) or they're just there to make you read more words in your pursuit of another decision point. If you don't use a comma-separated modifier to eliminate a sentence, then try ignoring it to make a shorter sentence.
Explanation Courtesy Brian from Veritas Prep